The main difference between Hoover UH70120 and UH70825 are the versions of WindTunnel technology they feature and color.
As you can see from the product name, Hoover UH70120 utilize WindTunnel technology while UH70825 features Windtunnel 2.
And the position of the ON/OFF switch is different. On/Off switch on the UH70120 is located on the handle that is easier to reach while the switch on the UH70825 is located on back of the unit.
For those who like the retractable cord, these 2 vacuum cleaners fit your needs, both of them have the cord rewind system.

Compare Hoover UH70825 and UH70120

	Hoover UH70120 WindTunnel T-Series Rewind Plus Bagless Upright Vacuum Cleaner	Hoover UH70825 WindTunnel® 2 Rewind Bagless Upright
Design	Upright	Upright
Price	$129.99 at Hoover.com on July 26, 2014	$129.99 at Hoover.com on July 26, 2014
Bagged or Bagless	Bagless	Bagless
Dirt Bin Capacity	not specified	not specified
Bottom-Release Bin	Yes	Yes
Cord Length	27 ft	25 ft
Auto Cord Rewind	Yes	Yes
Amps	12 Amps	12 Amps
Motorized Brushroll	Yes	Yes
Brushroll On/Off	No	No
Height Adjustment Type	Manual, 5 Settings	Manual, 5 Position Height Adjustment
Washable Filter	Yes	Yes
HEPA Filtration	Yes	Yes
Headlight	No	No
Weight	16.5 lbs	17.4 lbs
Cyclonic Action	Dual	Dual-Cyclonic
WindTunnel Technology	WindTunnel	WindTunnel 2
Performance Check Indicator	Yes	Yes
Nozzle Width	13.5"	13.5"
Edge Cleaning Brush	No	No
Self-propelled	No	No
Handle	D-Shaped Handle	D-Shaped Handle
Deep Recline Handle	No	No
Hose Type	stretch hose	stretch hose
Hose Length	8'	8'
Included Tools	Air Powered Hand Tool, Crevice Tool, Dusting Brush, Upholstery Tool	Air Powered Hand Tool, Crevice Tool, Dusting Brush
Cleaning Wand	Yes	Yes
Color	Gray and Green	Black
Warranty	2 year limited warranty	2 year limited warranty
Manual	download manual	download manual

Both of them do not have the switch to turn off the brush bar, when you want to vacuum hard floor, you can adjust the dial to bare floor setting.

Some main difference between Hoover UH70120 and UH70400 are:
1, There is no Manual height adjustment for floors on the UH70400, it uses an automatic height adjustment mechanism while the UH70120 use manual adjustment which has 5 options.
2, The UH70120 has cord rewind while the UH70400 does not have.
3, The UH70120 has no Brush Off switch while you can turn off brushroll for bare floor cleaning on UH70400.

For more details, read the side by side comparison table below:

Compare Hoover UH70400 and UH70120

	Hoover UH70120 WindTunnel T-Series Rewind Plus Bagless Upright Vacuum Cleaner	Hoover UH70400 WindTunnel® Air™ Bagless Upright
Design	Upright	Upright
Price	$129.99 at Hoover.com on July 24, 2014	$159.99 at Hoover.com on July 24, 2014
Bagged or Bagless	Bagless	Bagless
Dirt Bin Capacity		1.2 liters
Bottom-Release Bin	Yes	Yes
Cord Length	27 ft	30 ft
Auto Cord Rewind	Yes	No
Amps	12 Amps	11 Amps
Motor Watts		1200 Watts
Motorized Brushroll	Yes	Yes
Brushroll On/Off	No	Yes
Height Adjustment Type	Manual, 5 Settings	Automatic
Washable Filter	Yes	Yes
HEPA Filtration	Yes	Yes
Headlight	No	No
Weight	16.5 lbs	12.4 lbs
Cyclonic Action	Dual	Multi-Cyclonic
WindTunnel Technology	Yes	Yes
Nozzle Width	13.5"	11"
Edge Cleaning Brush	No	No
Self-propelled	No	No
Handle	D-Shaped Handle, Not Detachable, not Telescopic	Detachable Telescopic Handle
Deep Recline Handle recline flat to clean under furniture.	No	Yes
Hose Type	stretch hose	stretch hose
Hose Length	8'	12'
Included Tools	Air Powered Hand Tool, Crevice Tool, Dusting Brush, Upholstery Tool	3-in-1 Dusting, Crevice, and Upholstery Tool
Cleaning Wand	Yes	Yes
Color	Gray and Green	Gray
Warranty	2 year limited warranty	2 year limited warranty
Manual	download uh70120 manual	download uh70400 manual

Which one is better to buy?
If you need a lighter one, choose the UH70400; if you would like to get one that has retractable cord, then buy the UH70120.

1, Hoover Windtunnel Air vs Steerable
One of the key difference between Hoover UH72400 and UH70400 is that the UH72400 WindTunnel® Air Steerable is equipped with Steerable Technology which makes it easier to navigate around corners while the UH70400 WindTunnel® Air does not have.
Is that clear? If not, let’s get more details about the steerable design.
We can say that the UH72400 Steerable Air has a swivel head while the UH70400 Non Steerable Air has a fixed head. The swivel head is more maneuverable around objects. For example, when you clean around furniture, with the Steerable Air, you just need a wrist movement to change direction; with the Non Steerable Air, you have to change your body position and back up the vacuum to change direction constantly. Therefore, the Steerable Air will save you time and effort.

2, Another difference is Nozzle Width. Hoover UH72400 has a wider nozzle which is 12.5 inches in width while it’s 11 Inches on UH70400.

And UH72400 has a better design for the hose connection to prevent the hose from popping off.
The UH72400 also carries a longer warranty that is 5-year instead of 2 years on UH70400.

Read more specs comparison on Hoover UH70400 and UH72400 below:

Compare Hoover UH70400 and UH72400

	Hoover UH72400 Air™ Steerable Bagless Upright	Hoover UH70400 WindTunnel® Air™ Bagless Upright
Design	Upright	Upright
Steerable Technology	Yes allows you to navigate smoothly	No
Price	$159.99 at Hoover.com on July 23, 2014	$159.99 at Hoover.com on July 23, 2014
Bagged or Bagless	Bagless	Bagless
Dirt Bin Capacity	1.2 liters	1.2 liters
Bottom-Release Dirt Cup	Yes	Yes
Power Source	Corded	Corded
Cord Length	30 ft	30 ft
Auto Cord Rewind	No	No
Amps	11 Amps	11 Amps
Motor Watts	1300 Watts	1200 Watts
Motorized Brushroll	Yes	Yes
Brushroll On/Off	Yes	Yes
Height Adjustment Type	Automatic	Automatic
AllergenBlock prevent 99% of allergen particles from re-entering the air.	Yes	Not specified
Washable Filter	Yes	Yes
HEPA Filtration	Yes	Yes
Headlight	No	No
Weight	13.0	12.4
Cyclonic Action	Multi-Cyclonic	Multi-Cyclonic
WindTunnel Technology	WindTunnel® 3 Technology	WindTunnel® Technology
Nozzle Width	12.5"	11"
Edge Cleaning Brush	No	No
Self-propelled	No	No
Handle	Detachable Telescopic Handle	Detachable Telescopic Handle
Hose Type	stretch hose	stretch hose
Hose Length	12'	12'
Included Tools	Crevice Tool, Dusting Brush	3-in-1 Dusting, Crevice, and Upholstery Tool
Cleaning Wand	Yes	Yes
Color	Gray	Gray
Warranty	5 year limited warranty	2 year limited warranty
Manual	download uh72400 manual	download uh70400 manual

The difference between Hoover UH72400 and UH72450 is:
Hoover UH72400 is lighter and cheaper while the Hoover UH72450 has a larger dust bin and comes with a Turbo Tool for furniture and stairs cleaning.
And there is a Air Valve Dial on UH72450 that can be used to reduce suction power and makes switching between floor modes and tool modes easier by turning the dial. On the UH72400, the hose needs to be moved from one port to another to use the tools.
The comparison table below shows more details:

Hoover Air Steerable vs Pro

	Hoover UH72400 Air™ Steerable Bagless Upright	Hoover UH72450 Air™ Pro Bagless Upright
Design	Upright	Upright
Steerable Technology allows you to moves the vacuum around corners and objects smoothly	Yes	Yes
Price	$159.99 at Hoover.com on July 22, 2014	$199.99 at Hoover.com on July 22, 2014
Bagged or Bagless	Bagless	Bagless
Dirt Bin Capacity	1.2 liters	1.5 liters
Bottom-Release Dirt Cup	Yes	Yes
Power Source	Corded	Corded
Cord Length	30 ft	30 ft
Auto Cord Rewind	No	No
Amps	11 Amps	11 Amps
Motorized Brushroll	Yes	Yes
Brushroll On/Off	Yes	Yes
Height Adjustment Type	Self Adjust	Self Adjust
AllergenBlock prevent 99% of allergen particles from re-entering the air.	Yes	Yes
Washable Filter	Yes	Yes
HEPA Filtration	Yes	Yes
Headlight	No	No
Weight	less than 14 lbs	14.4 pounds
Cyclonic Action	Multi-Cyclonic	Multi-Cyclonic
WindTunnel Technology	WindTunnel® 3 Technolog	WindTunnel® 3 Technolog
Nozzle Width	12.5"	12.5"
Handle	Detachable Handle	Detachable Handle
Hose Type	stretch hose	stretch hose
Self-propelled	No	No
Edge Cleaning Brush	No	No
Included Tools	Crevice Tool, Dusting Brush	Air Powered Turbo Tool for cleaning furniture and stairs, Crevice Tool, Dusting Brush
Cleaning Wand	Yes	Yes
Hose Length	12'	12'
Above Floor Cleaning	up to 14 feet	up to 15 feet
Color	Gray	Gray
Warranty	5 year limited warranty	5 year limited warranty
Manual	download uh72400 manual	download uh72450 manual